precast building on top of cantilevered elevated deck slab 2

[delagina]

long story.

precast building by others ended up on top of a cantilevered slab.
i am getting 2' thick slab due to shear.

any idea how to increase shear capacity/reduce slab thickness in this situation.

i pretty much have an idea what to check but would want your inputs what else besides the shear, should i be concerned with this structure.

 

[JAE]

Don't forget that you also probably have lateral wind suction pulling outward on the slab - reducing your [φ]Vc.

Increase of shear capacity for slabs is either by increasing [red]d[/red] or [red]f'c[/red].

 

[BAretired]

1. Don't notch slab at precast wall.

2. Don't use separate top steel and make sure you continue it far enough to handle the worst case of negative moment across the span.

3. Shear is not critical on interior side of beam, so consider forming slab to bottom of beam on exterior side only, i.e. bear on bottom flange.

BA

 

[delagina]

@ba, can you be so kind as to sketch your 2 and 3.
i have to notch the slab. water might get inside.

 

[hokie66]

I agree with BA's approach, and don't see why you need a sketch. His word picture is crystal clear.

 

[BAretired]

delagina,

I attach a sketch showing what I suggested earlier. I have added bent bars passing close to the steel beam to carry shear.

BA

 

[SAIL3]

That is an interesting solution by BA, however, it still leaves me with a few questions.
It seems to me a very high price to pay in slab thickeness and
the support of this slab just to handle a load on a one foot
cantilever.
Would it be possible to run another edge beam underneath the
prefab wall and connect it to the WF(shown), say, every 5ft by
brackets.This would put torsion in that WF which could be reacted
by a backup bm at each brkt running from the WF to the next floor bm. The backup bm would be moment connected to the WF.
This setup could reduce the thickeness of the slab considerably
unless governed by other floor loads.
An overall question still remains about the deflection of the
local support and the prefab wall. I believe the stiffeness of the wall compared to the stiffeness of the support would not result in a linear loading on the floor. One would expect the wall to act as a deep girder and therby tend to shift the loads
to the vicenity of the ends where the support system is stiffer and could result in large loads in that area.

 

[FinnB]

25 kip/ft is a decent load. How many stories is the precast building above?

At 25 kip/ft the steel beam you show will be very big if it spans any kind of length. What does this steel beam span on to. Have you got any precast crosswalls perpendiular to the wall on the end of the cantilever that you show in the sketch? Cross walls could act as deep beams cantilevering out.

If you have columns supporting the steel beam could you place 1' span cantilevers off the side of the columns and then span the precast front wall between the steel cantilevers.

 

[delagina]

i actually made an error with 25kip/ft. i FEM modeled it in staad with support every 4 ft. so it's just about 6 kip/ft.

i believe this is typical precast building for the vendor where wall is continuously supported on slab on grade. in this case though it's on elevated slab. i doubt wall can be supported every column.

 

[csd72]

I would analyse that cantilever as a strut and tie rather than a beam,